1. Field of the Invention
Reduction of radioactivity in clays.
2. Description of the Prior Art
Kaolin is a form of clay which has been used for many purposes because of its whiteness and relative purity. Certain kaolins, such as Georgia kaolins are known to have a very low heavy metal content, that is to say, less than 10 parts per million of heavy metals such as mercury, arsenic lead, cadmium, and selenium. The kaolins have therefore found use in applications such as pharmaceutical preparations including antidiarrhetics, toothpaste, cosmetics, and food wrapping materials. Such uses continue since Georgia kaolins heretofore were believed to meet the FDA maximum tolerance for heavy and toxic metals in material coming into contact with humans or human food at 10 parts per million.
It is now known that kaolins contain significant radioactive materials. These radioactive materials fall into three general categories.
The first category includes heavy metal natural radiogenic elements such as uranium, thorium, radium and their daughter isotopes; these elements are of course usually found as compounds with other elements.
Secondly, radioactivity may be due to fallout materials from nuclear testing which includes relatively short lived isotopes and some relatively heavy metal isotopes.
Thirdly, radiogenic potassium is found to occur in one part per 10 thousand of natural potassium.
Analysis of kaolin samples from different locales shows a considerable variation in the amount and energy distribution of the emissions, thus indicating inconsistencies in the origin of the radioactivity. Of all three sources indicated the least hazardous is the radiopotassium, K.sub.40, since its occurrence is of the order of between 0.1 and 0.02 parts per million in the kaolins. Furthermore, potassium is an element which is excreted fairly rapidly from the human body and therefore does not constitute a long range hazard.
Heavy metal compounds and fall-out radiogenic materials can, if present in sufficient amounts, constitute a substantial health hazard since once absorbed by the body they tend to accumulate therein. Thus, in the case of anti-diarrhetics which are compounded with kaolin, there is a direct intake into the digestive systems and, due to the chemical, and particularly the acid nature of the stomach fluids, there is a tendency to dissolve such compounds whereby they are converted into a condition which permits them to be absorbed through the membranes of the body. Similarly, when used as cosmetics, the chemical nature of the skin may, over a long period of time, leach such elements into the body's membranes. In addition, long term contact with radiogenic clays will, itself, produce radiation damage to tissues. Heretofore, kaolin has not been considered as a material giving rise to radiation hazards.
Recently the Atomic Energy Commission pursuant to the Occupational Safety and Hazards Act, has set certain maximum radiation exposure levels to external materials. The permissible radiation exposure is measured in units of radiation damage to the body produced by beta, gamma, or both forms of radiation and designated as the rem. The permissible standards have been set at 1.25 rems per 3 months with respect to workers and other personnel wearing protective shielding and 0.125 rems where the exposee does not wear protective equipment.